1. Field of the Invention
The present invention relates to polysiloxazanes, silicon oxynitride fibers, and processes for producing the same. Polysiloxazanes, when formed into fibers, are useful among others as the precursor of silicon oxynitride fibers, which are useful for the reinforcement of various composite materials strongly demanded by many industries, including those related to transportation, energy, metals and aerospace.
2. Description of Related Art
New ceramic materials such as silicon carbide and silicon nitride have been attracting much attention for their favorable properties, and the research thereof has recently made remarkable development. Silicon oxynitride is another type of new ceramic material, which is known to be as heat-resistant as, and more oxidation-resistant than, silicon carbide and nitride. Some of more important processes, among a number of others which have been proposed for the synthesis of silicon oxynitride, are:
(1) A process wherein a mixture of metallic silicon and silicon dioxide as the starting material is nitrided in the presence of an alkaline earth metal or alkali metal fluoride in a nitrogen atmosphere at 1150 to 1350.degree. C. (Japanese Unexamined Patent Publication (Kokai) No. 47-42400/1972 by Tada). In another process, another metal for example, iron, copper, manganese, nickel, magnesium or aluminum, is added to the feed mixture of metallic silicon and silicon dioxide, which is heated in a nitrogen atmosphere to form discontinuous fibers or whiskers (Japanese Unexamined Patent Publication (Kokai) Nos. 50-29498/1975, 51-129898/1978 and 53-79799/1978, all by Azuma).
(2) A process, wherein polycarbosilanes are formed into fibers, which are treated, after being cured by oxidation, with ammonia at 800 to 1400.degree. C to form continuous fibers of silicon oxynitride having a structures of SiN.sub.1.5 O.sub.0.47. (Okamura et al, Chemistry Letters, pp, 2059 to 2060, 1984).
The above process (1) is, however, not a process which provides a continuous fiber of oxynitride. The ability to form a continuous fiber is extremely advantageous, since not only oxynitride but also other ceramic materials have a remarkably increased mechanical strength and provide an increased shapeability when formed into a fibrous form.
The above Process (2) is a process which provides continuous fibers of silicon oxynitride. However, the process is complex and there is a strong desire for an improvement of this process.